Decomposable (probabilistic) models are log-linear models generated by acyclic hypergraphs, and a number of nice properties enjoyed by them are known. In many applications the following selection problem naturally arises: given a probability distribution $p$ over a finite set $V$ of $n$ discrete variables and a positive integer $k$, find a decomposable model with tree-width $k$ that best fits $p$. If $\mathscr{H}$ is the generating hypergraph of a decomposable model and $p_{\mathscr{H}}$ is the estimate of $p$ under the model, we can measure...

We characterize the class [...] L32 $L_3^2$ of intersection graphs of hypergraphs with rank at most 3 and multiplicity at most 2 by means of a finite list of forbidden induced subgraphs in the class of threshold graphs. We also give an O(n)-time algorithm for the recognition of graphs from [...] L32 $L_3^2$ in the class of threshold graphs, where n is the number of vertices of a tested graph.

A graph is diameter-2-critical if its diameter is two and the deletion of any edge increases the diameter. Let G be a diameter-2-critical graph of order n. Murty and Simon conjectured that the number of edges in G is at most ⌊n 2/4⌋ and that the extremal graphs are the complete bipartite graphs K ⌊n/2⌋,⌊n/2⌉. Fan [Discrete Math. 67 (1987), 235–240] proved the conjecture for n ≤ 24 and for n = 26, while Füredi [J. Graph Theory 16 (1992), 81–98] proved the conjecture for n > n 0 where n 0 is a...

Whitney’s Broken-cycle Theorem states the chromatic polynomial of a graph as a sum over special edge subsets. We give a definition of cycles in hypergraphs that preserves the statement of the theorem there

For a digraph $D$, the niche hypergraph $N\mathscr{H}\left(D\right)$ of $D$ is the hypergraph having the same set of vertices as $D$ and the set of hyperedges $E\left(N\mathscr{H}\right(D\left)\right)=\{e\subseteq V(D):|e|\ge 2$ and there exists a vertex $v$ such that $e=N_D^{-}\left(v\right)$ or $e=N_D^{+}\left(v\right)\}$. A digraph is said to be acyclic if it has no directed cycle as a subdigraph. For a given hypergraph $\mathscr{H}$, the niche number $\widehat{n}\left(\mathscr{H}\right)$ is the smallest integer such that $\mathscr{H}$ together with $\widehat{n}\left(\mathscr{H}\right)$ isolated vertices is the niche hypergraph of an acyclic digraph. C. Garske, M. Sonntag and H. M. Teichert (2016) conjectured that for a linear hypercycle...

We prove that a k-uniform self-complementary hypergraph of order n exists, if and only if $\left(\genfrac{}{}{0pt}{}{n}{k}\right)$ is even.

In this paper, we establish a theorem on Möbius inversion over power set lattices which strongly generalizes an early result of Whitney on graph colouring.

A 2-packing of a hypergraph 𝓗 is a permutation σ on V(𝓗) such that if an edge e belongs to 𝓔(𝓗), then σ (e) does not belong to 𝓔(𝓗). We prove that a hypergraph which does not contain neither empty edge ∅ nor complete edge V(𝓗) and has at most 1/2n edges is 2-packable. A 1-uniform hypergraph of order n with more than 1/2n edges shows that this result cannot be improved by increasing the size of 𝓗.

For an integer $k\ge 2$ and a $k$-uniform hypergraph $H$, let ${\delta }_{k-1}\left(H\right)$ be the largest integer $d$ such that every $(k-1)$-element set of vertices of $H$ belongs to at least $d$ edges of $H$. Further, let $t(k,n)$ be the smallest integer $t$ such that every $k$-uniform hypergraph on $n$ vertices and with ${\delta }_{k-1}\left(H\right)\ge t$ contains a perfect matching. The parameter $t(k,n)$ has been completely determined for all $k$ and large $n$ divisible by $k$ by Rödl, Ruci’nski, and Szemerédi in [Perfect matchings in large uniform hypergraphs with large minimum collective degree, submitted]....